SAMD51 apparently has a separate MCLK for running the CPU itself instead of running it off of GCLK0? Or not? I'm not quite understanding the datasheet or code, and I've run into things before where some things run off arbitrary GCLKs, some things run off of specific GCLK, and some things run off of "other" clocks (notably, the RTC runs directly from a 32kHz oscillator, except on SAMD10 (where it runs off of a gclk.)

we don't have any more details or docs for that library - but yes we like to keep it SAMD21 compatible. if you have any updates, send a PR - the library is not well supported right now, we don't advertize it or anything :)